One subpopulation of mitochondria in skeletal muscle is located immediately beneath the sarcolemma.

• A. Around the contractile proteins
• B. Immediately beneath the sarcolemma
• C. Near the nucleus
• D. In the sarcoplasmic reticulum

Answer: (B)

The main idea is that skeletal muscle has specialized mitochondrial groups positioned to meet different local energy needs. The subset located immediately beneath the sarcolemma is called subsarcolemmal mitochondria. Their proximity to the cell membrane and its ion pumps, channels, and surface signaling means they can rapidly supply ATP where it’s most needed at the surface, supporting processes like membrane transport and initial steps of excitation–contraction coupling that occur near the sarcolemma. Other mitochondria sit elsewhere to serve different tasks—those surrounding the contractile apparatus provide energy for muscle contraction, while perinuclear mitochondria support energy needs for gene expression and nuclear processes. The idea that mitochondria reside inside the sarcoplasmic reticulum isn’t accurate; ATP for Ca2+ handling in the SR comes from cytosolic mitochondria and other cellular sources rather than mitochondria embedded within the SR membranes.